Help! Loosing boost at high altitudes!

[Martens199]

Yeah threads where shot in the tube. But as I found out today there is more to this problem, I had lost my boost today again . Removed the mounting bracket and screwed in bolt completely and then covered with sealant and tape. On the mountain fixes, yay nothing better lol. But still nothing. Once again going up the hill I was boosting and also when coming down. Although yesterday I drove up the hill constantly testing to check if i still had boost and when the boost cut out ( which it didn't where it did before). But today when I drove up I just keep a steady pace and I lost boost when I got to the top. Not sure if this on and off throttle could have helped me start boosting like it did, but I thought I should share it.

 

[rxrider]

There is another possible cause to your boost drop.
Your turbo bearings could be shot, causing the compressor wheel to stall, causing the boost drop.

 

[Martens199]

Very good point but if that where the case wouldn't it always be intermittent?
I have been riding the sled back on flat ground for the last few weeks and sled has been boosting constantly. Except for the other day it started boosting low at 4pounds, then a couple miles farther it went back up again.
Would this be worth tring to replace the TCV valve? Is there any tests I can do to check this valve and even maybe the waste gate pot?

 

[rxrider]

The TCV valve cannot cause the boost to drop below the wastegate setting.
Remember, at altitude the turbo will spin way higher rpms to maintain boost pressure because of the thin air. Maybe the high rpms causes the bearings to seize up ...... it's just a theory, but who knows..... anyway if what I'm thinking is what's happening, the turbo bearing will eventually give away and the compressor turbine blades will destroy themselves and the compressor housing, probably the exhaust blades as well.

Hope you find the problem.

 

[Martens199]

Are these turbo bearings serviceable and sold separately? I know most of the equipment I work on the entire turbo would have to be replaced?
I will inspect the turbo this weekend, see if I see any oil seeping into the intake or exhaust. Also is there a measurement for free play on the impeller shaft?

 

[Martens199]

Inspected the turbo and found that there had been no visual damage. Impeller spins freely and there is very little to any up and down movement. Removed the plenum and retreaded the bolt hole that stripped out in the mountains. When reinstalling made sure to really check if I got the boosts on tight and lined up correctly.
Found on the turbo that the actuating arm the connects to the waste gate was built up with some corrosion/rusty stuff so I pulled the arm out as much as I could and sanded smooth. My thoughts where that this could have caused it to bind? But it wouldn't have caused it to leak because its on the external side of the diaphragm... Correct? It not that familiar on how those work on the inside but I believe it's just a spring loaded rubber diaphragm that is pushed out with air, so there would be no seal riding on this shaft?? Sound about right?
Anything else I can try?

 

[Martens199]

Still a sitting duck with this boost problem.

Hey guys,
So I recently pulled the sled out in order to get it ready for the up coming year. At the end of last year I rode the sled endless days once I returned from my trips to the mountains (were any time over an est 6000ft I would loose all boost). Throughout the days the sled ran flawless. But I am still nervous about this high altitude boosting problem. I have went through the machine to the best of my knowledge so far and have not found any positive results. I just recently picked up a new boost gauge one that reads into negative pressures (vacuum). I was told that running my machine at idle I should see around 7 inHg. If so my intake side of the machine did not have an air leak. So I did this with my sled idling and yes I did have around 8 inHg.
Does this sound right to you guys?
I know it really sucks to diagnose a problem that cannot be truely tested, but any help would be awesome, places to check or past problem etc.

 

[Dimebag]

I see about -0.3 bars on my boost gauge at idle. So your readings sound correct.

 

[Martens199]

Ok so Im pretty sure I have pin pointed it to being the TVC valve. Thinking of replacing it, but also here that the new supplier for this parts aren't as good anymore and they have been having problems with the TVC valves. So I am swaying towards a manual....

 

[Dimebag]

The tcv never worked as supposed for me. A manual boost controller is the key. I really like my hallman pro mbc

 

[rxrider]

Agree With Dimebag, I replace the TCV whenever I build a MCX sled With higher boost than the Stock MCX boost.

I'm not convinced the TCV is the culprit, nor the wastegate actuator arm. A malfunctioning TCV or stuck WG arm will make you overboost, unless the WG-arm gets stuck With the WG more or less open.

Do a pressure test on the charge tube, spray the CT With soapy water, pressurize to the boost Level of choise and check for leaks.

 

[Martens199]

Yeah come to think of it thats very true. It the TVC is not functioning and at its rest position nothing will limit my boost because the turbo wastegate would be closed correct?
How would I got about pressure checking the charge side?

 

[Martens199]

I was taking to Don from MCXpress and after checking my vacuum on my sled and it was in spec. The only thing he thought possible or for me to try was the TVC, he seemed postive on that. But I do see where your coming from on this one rxrider.

 

[Martens199]

Sorry spelling error.. How do you do a pressure tests on these systems? Is it as simple as suppling air throught the intake and in through the exhaust?

 

[rxrider]

You may PM member TT as he has done it.
You may have a look in his thread in the TY 4stroke Garage section.